Alkenes frans-disubstituted

Ruasse et al, 1978) is totally regioselective and shows X-dependent chemoselectivity. This is partly in agreement with the kinetic data, which indicate no primary carbocation but rather a competition between the benzylic carbocation and the bromonium ion, depending on X. According to the data of Table 6, bridged intermediates would lead to more dibromide than open ions do. From these results and from those on gem-, cis- or frans-disubstituted alkenes, empirical rules have been inferred for chemoselectivity (i) more solvent-incorporated product is formed from open than from bridged ions (ii) methanol competes with bromide ions more efficiently than acetic acid. 

This reaction is useful for asymmetric hydroboration of m-disubstituted alkenes to give highly pure optically active alcohols, but is less useful in the case of 1,1-disubstituted, frans-disubstituted, or trisubstituted alkenes. 

C—I I)tran,s 965 Irons Fat This is a great band for monitoring frans-disubstituted alkene groups in unsaturated fat. 

An 11 kJ/mol (2.7 kcal/mol) stability difference is typical between a monosubstituted alkene (but-l-ene) and a trans-disubstituted alkene (frans-but-2-ene). In the following equations, we compare the monosubstituted double bond of 3-methylbut-l-ene with the trisubstituted double bond of 2-methylbut-2-ene. The trisubstituted alkene is more stable by 14 kJ/mol (3.4 kcal/mol). 

Substrate Scope. Best results in the (salen)Mn -catalyzed epoxidation reaction have been obtained with cis-disubstituted, conjugated alkenes (Table 1). Epoxidation of 2,2-dimethylchromene derivatives occurs with especially high selectivity (>97% ee). frans-Disubstituted alkenes are epoxidized with low selectivity (20-50% ee), as are simple alkyl-substituted alkenes. 

Davies has provided a detailed analysis of how the arrangement of four proli-nate ligands around a dirhodium core can lead to a metal carbene intermediate that reacts further with such high stereocontrol [12]. The two most striking features of the vinyl diazoester cyclopropanations are the excellent diastereoselec-tivity and the total lack of reactivity of frans-disubstituted alkenes. The former is accounted for by the structure of the vinyl diazoester which suggests that high... 

The ene reaction where both partners are simple alkenes is quite useful when the partners are linked so as to provide for an intramolecular ene reaction. There is a preference for the formation of cw-disubstituted cyclopentane and frans-disubstituted cyclohexane systems resulting from thermally induced, intramolecular ene reactions. For example, levels of transjcis control of 92 8 to > 99 1 were found for the intramolecular cyclization of methyl 2-cyano-9-methyl-3,8-decadienoate23 24. Furthermore, the same tran. -stereochemical relationship was obtained when the reaction was carried out at room temperature in the presence of zinc(II) bromide with control consistently at the 99 1 level for R and R2 either methoxycarbonyl or cyano groups24, This preference for trans fusion does not appear to be greatly altered by added substituents on the chain connecting the reacting partners (vide infra). 

Simple Alkyl-Substituted Alkenes. The C=C stretehing frequency usually appears between 1670 and 1640 cm for simple noncyclic (acyclic) alkenes. The C=C frequencies increase as alkyl groups are added to a double bond. For example, simple monosubstituted alkenes yield values near 1640 cm 1,1-disubstituted alkenes absorb at about 1650 cm and tri- and tetrasubsti-tuted alkenes absorb near 1670 cm". frans-Disubstituted alkenes absorb at higher frequencies... 

A Cu(I)-7V-heterocyclic carbene complex, IPrCu(DBM), catalyzes alkene aziridination with TcesNH2, Phl=0, and 4 A molecular sieves (eq 4). Reactions are performed with limiting alkene as- and frans-disubstituted olefins give isomeric product mixtures. In one example with styrene, TcesNH2, and PhI(OAc)2, 3 mol % of an Au(I) catalyst, [Au( Bu3tpy)](OTf), is found to catalyze aziridine formation. ... 

SCHEME 28.13 Hydroboration of fran -disubstituted and trisubstituted alkenes with IpcBHj,... 

Enantioselective osmylotion of alkenes. Osmium tetroxide forms a 1 1 wine-red complex with the chiral diamine l2 that effects efficient enantioselective dihy-droxylation of monosubstituted, oww-disubstituted, and trisubstituted alkenes (83-99% ee) at -110° in THF. The enantioface differentiation in all cases corresponds to that observed with t/mr-3-hexene and the complex with (-)-l. Essentially complete asymmetric induction is observed with frans-l-phenylpropene (99% ee). 

The absorption of symmetrical disubstituted trans-alkenes or tetrasubstituted alkenes may be extremely weak or absent. The c/s-alkenes, which lack the symmetry of the trans structure, absorb more strongly than frans-alkenes. Internal double bonds generally absorb more weakly than terminal double bonds because of pseudosymmetry. 

C C Stretching Vibrations Unconjugated Linear Alkenes The C=C stretching mode of unconjugated alkenes usually shows moderate to weak absorption at 1667-1640 cm-1. Monosubstituted alkenes, that is, vinyl groups, absorb near 1640 cm"1, with moderate intensity. Disubstituted frans-alkenes, tri- and tetraalkyl-substituted alkenes absorb at or near 1670 cm"1 disubstituted dr-alkenes and vinylidene alkenes absorb near 1650 cm"1. 

In Chapter 7, we used heats of hydrogenation to compare the relative stabilities of alkenes. For example, the heats of hydrogenation of pent-l-ene and fran.s-pent-2-ene show that the disubstituted double bond in trans-pent-2-ene is 10 kJ/mol (2.5 kcal/mol) more stable than the monosubstituted double bond in pent-l-ene. 

Application of Various Chiral Reagents Derived from IpciBH. Diisopinocampheylborane does not normally yield satisfactory ee s in hydroboration reactions of 1,1-disubstituted alkenes, /rans-alkenes, or trisubstituted alkenes. This problem has been partially solved by the introduction of Monoisopinocam-pheylborane, IpcBHa, which is derived from IPC2BH. IpcBH2 handles frans -alkenes and trisubstituted alkenes effectively, since... 

S-methyl dithiocarbonate (76S413). Stereoselective isomerization of 1,2-disubstituted alkenes may be achieved by a sequence such as the following frans-alkenebromo-hydrin -hydroxythiocyanate cw-thiirane cw-alkene (75TL2709). 

CH2I2 / ether R l 1,2-c/s-Disubstituted cyclopropane - ( )-1,2-disubstituted alkene CH2I2 / ether R1 1,2-frans-Disubstitu-ted cyclopropane... 

The enantioselectivities obtained in the first catalytic AD process (Table 2) reveal that the best results are obtained with trans-substituted alkenes with at least one aromatic substituent (entries 2 and 3). If the aromatic substituent is moved further away from the alkene, the ee drops sharply (entry 6). Furthermore, terminal alkenes give moderate ee provided a directly bond aryl group is present (entry 1). 1,1-Disubstituted (entry 4) and frans-dialkyl-substituted alkenes... 

The E/Znomenclature was introduced in Chapter , and rtdyv) that you have read Chapter 16 we can be more precise with our definition. For disubstituted alkenes, Ecorrespondsto tran and Zcorresponds to cis. To assign Eor Zto tri-or tetrasubstituted alkenes, the groups at either end of the alkene are given an order of priority according to the same rules as those outlined for f and Sin Chapter 16. If the two higher priority groups are c/s, the alkene is Z if they are frans the alkene is Of course, molecules don t know these rules, and sometimes (as in the second example here) the Ealkene is less stable than the Z. 

With a few exceptions, 1,2-disubstituted alkenes are not polymerized because of steric hindrance. The exceptions include 1-deuteropropene (Sec. 8-4g) and cycloalkenes. Polymers are obtained from some 1,2-disubstituted alkenes, but the reactions involve isomerization of the monomer to a 1-alkene prior to polymerization, e.g., 2-butene yields poly( 1-butene) [Endo et al., 1979]. There is one report of polymerization of frans-2-butene to poly(frans-2-butene) using the a-diimine nickel initiators described in Sec. 8-8b [Leatherman and Brookhart, 2001]. 

In the osmylation of 1,2-disubstituted allylic alcohols and derivatives, cw-alkenes provide higher diastereoselectivity than the corresponding fran -alkenes (eqs 7 and 8). Opposite se-lectivities have been observed in the osmylation of (Z)-enoate and ( )-enoate esters (eqs 9 and 10). High selectivity has also been observed in the osmylation of 1,1-disubstituted and ( )-trisubstituted allylic alcohols and derivatives and bis-allylic compounds (eqs 11-13). 

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